(1) Field of the Invention
The present invention relates to an optical space switch that is used to set up a path between a fiber-optic transmission path on the incident side and a fiber-optic transmission path on the output side.
(2) Description of the Prior Art
With recent advances in the development of information transmission systems using optical fibers as information transmission paths, the need has been increasing particularly for a polarization control optical space switch that performs switching to direct light information in the form of a light signal, without converting it into an electrical signal, from an input fiber-optic transmission path to a selected output fiber-optic transmission path.
FIG. 31 is a diagram showing the configuration of a prior art polarization control optical space switch.
This polarization control optical space switch has n inputs and n outputs, and performs light path switching for light information which entered as p-polarized light.
The polarization control optical space switch shown comprises n.sup.2 switch elements, SW11-SWnn, arranged as a matrix of n rows and n columns.
On the input side of this polarization control optical space switch are arranged n input fibers Ii (i=1, 2, . . . , n). Furthermore, a lens L and a polarizer Pi (i=1, 2, . . . , n) are arranged between each input fiber Ii and each input light path to the polarization control optical space switch.
The lens L is a converging lens that converges the light information emerging from the input fiber.
The polarizer Pi is an element that allows light information which entered as p-polarized light to pass through it.
Lenses L, the number of which is equal to the number of output light paths, are arranged on the output side of the polarization control optical space switch. On the output side of the lenses L, there are arranged n output fibers Oi (i=1, 2, 3, . . . , n), one for each lens L.
The light information output from the input fiber Ii is converged by the lens L and enters the polarizer Pi.
If the incident light information is p-polarized light, the light information is allowed to pass through the polarizer Pi and enters the first row of switch elements SWi1 (i=1, 2, . . . n).
Light information output from the n-th column of switch elements SWnj (j=1, 2, . . . , n) is converged by the lens L and enters the output fiber Oi.
In the above configuration, each switch element SWij is constructed from a combination of a polarization splitter and two polarization control elements formed from liquid crystals. The two polarization control elements are placed on the incident and reflected sides, respectively, of the polarization splitter.
The polarization splitter transmits incident light information in the rectilinear forward direction when its polarizing direction is p-polarization, and reflects incident light information in a vertical direction when its polarizing direction is s-polarization.
The structure is such that an external voltage can be applied as desired to the polarization control elements.
The polarization control elements each function to retain the polarizing direction of the incident light information when no voltage is applied, and to rotate the polarizing direction of the incident light information through .pi./2 when voltage is applied.
For example, consider a case in which the light information incident on the switch element SW11 is to be passed to the switch element SW12. Since the light information incident on the switch element SW11 is p-polarized light, voltage is not applied to the polarization control element on the incident side. In this case, the light information incident on the switch element SW11 first enters the polarization control element on the incident side. The light information with its p-polarization state retained is passed through the polarization control element on the incident side and enters the polarization splitter.
The polarization splitter transmits the incident p-polarized light in the rectilinear forward direction for input to the switch element SW12.
On the other hand, if the light information incident on the switch element SW11 is to be directed to the switch element SW21, voltage is applied to the polarization control elements on both the incident and reflected sides. In this case, the p-polarized light incident on the switch element SW11 first enters the polarization control element on the incident side. The polarization control element on the incident side then rotates the incident p-polarized light to convert it into s-polarized light which is input into the polarization splitter.
The polarization splitter reflects the incident s-polarized light vertically downward for input into the polarization control element on the reflected side.
The polarization control element on the reflected side rotates the incident s-polarized light to convert it into p-polarized light which is input to the switch element SW21.
The switch element SW21 then allows the p-polarized light incident from the switch element SW11 to pass through it, so that the light is directed to the switch element SW31. The p-polarized light is thus input to the switch element SWn1.
The switch element SWnl transmits the incident p-polarized light in the rectilinear forward direction, directing the light to the output fiber 01. Thus, by applying a voltage to the polarization control elements on both the incident and reflected sides of the switch element SW11, a path is set up between the input fiber I1 and the output fiber 01.
By externally controlling the polarization control elements of each switch element SWij in this manner, a path can be set up between a desired input fiber and output fiber.
In the prior art polarization control optical space switch, since each switch element is formed at an intersection of the matrix, two polarization control elements must be controlled per switch element when setting a connection path.
The prior art polarization control optical space switch has the further problem that the number of switch elements for light to pass through varies depending on the path to be set, resulting in differences in the transmission loss and crosstalk from path to path.
In view of the above problems, it is an object of the present invention to provide a polarization control optical space switch wherein the number of switch elements for light to pass through is always the same independently of the path to be set, thus suppressing differences in the transmission loss and crosstalk.